This invention relates to a device for centring the rim of a wheel on the wheel-holder unit of a tyre changer machine.
This invention also relates to a wheel-holder unit for a tyre changer machine comprising said device and the corresponding tyre changer machine.
The invention applies to the automotive technical field and, more specifically, to the sector of vehicle service equipment and tyre changer accessories.
In prior art wheel changer machines the wheel (in particular the rim of the wheel) is positioned by an operator on the wheel-holder unit (or tabletop) and, only then is centred relative to it by means of a clamping tool (or rod) equipped with a longitudinal shaft partly surrounded by a tapered portion (a device of that type is known from patent application EP 1157861 by this Applicant).
More precisely, during an initial step the rim is rested on the wheel-holder unit (which forms a horizontal supporting surface) by the operator, without any help (mechanical or other) which allows him to position the central hole of the rim so that it is coaxial with the central hole of the wheel-holder unit.
It is only at a later stage, during insertion of the above-mentioned shaft through the central holes of the rim and the wheel-holder unit that the rim is centred by the tangential action (that is to say, at a right angle to the direction of insertion) imparted by the tapered portion to the edges of the hole in the rim.
Consequently, the rim is translated tangentially on the wheel-holder unit from an off-centre position to a centred position by means of sliding of the hub of the rim on the wheel-holder unit itself.
Disadvantageously, that centring method is often subject to errors, since the friction force established between the rim and the wheel-holder unit tends to prevent sliding of the rim on the wheel-holder unit.
Indeed, sliding between the rim and the wheel-holder unit is only possible when the tangential component of the clamping force, imparted by the clamping device, is greater than the friction force between the rim and the wheel-holder unit, also depending on the clamping force.
In other words, the shape of the tapered portion transmits a fraction of the clamping force in a direction at a right angle to the direction of insertion of the tapered portion, that is to say, tangentially to the wheel-holder unit, so as to centre the rim.
However, when said tangential fraction is less than the friction force (generated by the clamping force multiplied by the friction coefficient between the rim and the wheel-holder unit), the rim can no longer slide on the wheel-holder unit.
Said problem is particularly evident when the operator places the rim in a position particularly offset from the central axis (that is to say, the axis of rotation) of the wheel-holder unit.
It should also be noticed that sliding between the rim and the wheel-holder unit tends to consume the surface of the rim.
This is particularly significant in operations for mounting or demounting tyres on or from rims of the “inverted” type, that is to say, having the extraction channel located in a position distal from the central hub compared with conventional rims (i.e.: the extraction channel is located close to an edge of the rim which is distal from the hub).
“Inverted” rims are mounted in the opposite way to conventional rims, that is to say, with the hollow facing upwards rather than towards the wheel-holder unit.
In light of this, it is clear how the surface on which the “inverted” rims rest is that which, during use, remains visible and consequently is of particular aesthetic value.
Therefore, it is clear that sliding of said surface on the wheel-holder unit during rim centring causes wear on the surface in a zone which, by its nature, must be kept integral and whose attractive appearance must be maintained.
For that purpose, in the prior art there is a centring device which overcomes said disadvantage.
Said device (or centring flange) comprises a flat disk from which multiple protuberances rise, arranged along a circle, for insertion in the rim mounting holes.
Consequently, the above-mentioned centring device can be coupled to the rim coaxially with it in such a way that they do not slide relative to each other. In practice, said device is interposed between the hub of the rim and the wheel-holder unit, thus avoiding deterioration of the rim during centring.
During centring it is the flat disk, and not the hub of the rim, that slides on the wheel-holder unit.
Disadvantageously, said device is particularly laborious to use since, before mounting it, precise adjustment is needed depending on the type of rim to be fitted.
In other words, the prior art centring device requires a set of preliminary operations to prepare it for mounting on the rim.
First, the distance between the mounting holes of the rim to be positioned on the wheel-holder unit must be measured, as well as the diameter of the circle along which said holes are distributed.
Then, the operator must adjust the device (and in particular the protuberances) depending on the information gathered, so as to put it in the correct configuration.
Only after said preliminary operations can the operator mount the device on the rim and position the assembly on the wheel-holder unit.
However, said device does not allow any guarantee of correct rim centring, since, as already indicated, if there is a significant offset between the rim (and the device) and the wheel-holder unit, the centring cone may not guarantee complete sliding of the rim relative to the wheel-holder unit.
It should be noticed that further prior art devices are known which can be interposed between the rim and the wheel-holder unit, although having aims and functions which are different to those of the centring device just described.
In particular, patent document IT1996VR00090 shows a tyre changer machine in which the wheel-holder unit is equipped with a clamping device which complements the tapered clamping tool.
More precisely, said device comprises a pair of disks which are concentric and superposed in such a way that they can rotate relative to each other and each having a groove in which spacer means are housed.
During rotation of the wheel-holder unit, the two disks are driven to rotate relative to each other and the spacer means (bearings or projections made in one piece with a disk) are brought out of the groove, forcing detachment of the two disks and increasing the clamping force on the rim.
It should be noticed that the two disks are constrained to remain coaxial with each other (i.e.: cannot be offset from or misaligned with each other) since they are coupled at respective through holes which allow the clamping tool to pass through them.
A very similar device is shown in patent document VR2005A00059.
As can be seen, the above-mentioned devices cannot be used as centring devices because they do not allow any radial movement (that is to say, at a right angle to the axis of rotation of the tabletop) of the rim.
In contrast, said devices are designed solely to increase the clamping force on the rim by means of an axial movement (that is to say, parallel with the axis of rotation of the tabletop) of one disk relative to the other, and in particular thanks to a movement of the upper disk (on which the rim rests) away from the lower disk (anchored to the machine frame).
U.S. Pat. No. 3,958,618A also shows a pair of elements abutted to each other and each able to rotate relative to the other. However, said device is applied to “heavy” tyre changer machines, that is to say, those having an axis of rotation of the wheel-holder unit which is substantially horizontal and therefore not subject to the problems covered in this document.